1. Technical Field of the Invention
The present invention relates to cellular telephone systems and, in particular, to a method and apparatus for effectuating quality driven voice channel selection during call set-up and hand-off in response to signal strength measurements made on idle voice channels.
2. Description of Related Art
Cellular telephone systems divide a large service area into a number of smaller discrete geographical areas called "cells" each typically ranging in size from about one-half to about twenty kilometers in diameter. Each cell is contiguous with multiple adjacent cells to provide continuous coverage throughout the service area. A base station including a plurality of transceivers capable of operating independently on different radio frequency channels is provided for each of the cells. Via the transceivers, the base stations engage in simultaneous communications with plural mobile stations operating within the area of the associated cell. The base stations further communicate via data links (and voice trunks) with a central control station, commonly referred to as a mobile switching center, which functions to selectively connect telephone calls to the mobile stations through the base stations and, in general, control operation of the system.
Each cell is assigned to use a predetermined set of voice frequencies. The availability of multiple voice frequencies per cell permits base stations to simultaneously handle many telephone conversations with many mobile stations. The voice frequencies assigned to a cell are preferably spaced apart across the frequency spectrum of the cellular band. This serves to minimize the instances of adjacent channel interference.
Because only a limited number of voice frequencies are available in the cellular band, the same voice frequencies are repeated (i.e., reused) in other cells in a distant part of large service areas with many cells. No adjacent cells, however, are assigned the same voice frequency. Furthermore, the power levels of the signal transmissions on any given voice frequency are limited in strength. The foregoing precautions serve to minimize the likelihood of co-channel interference caused by reuse of that same voice frequency in a distant cell.
In spite of the precautions taken by service providers, it is known that instances of adjacent channel and co-channel interference do occur. This interference often adversely affects system operation by, for example, degrading voice quality on the voice channels or interfering with the transmission and reception of control signals on the control channels.
The mobile switching center functions to dynamically allocate analog voice channels (comprising the assigned voice frequencies) and digital voice channels (comprising time slots in assigned digital time division multiple access (TDMA) voice frequencies) available in any one cell among the plurality of mobile stations located within the cell area that desire communications. Commands that allocate for mobile station use a certain analog or digital voice channel assigned to a given cell are transmitted from the mobile switching center to the base station for that cell. The commands are then relayed by the base station to the certain mobile station over a control channel to direct mobile station selection of the allocated voice channel for handling the call.
Allocation by the mobile switching center of a particular analog or digital voice channel in a cell to a particular mobile station for a cellular communication (i.e., a call) primarily occurs in two instances. The first instance is at call set-up when the subscriber activates the mobile station to initiate a call and the system selects the voice channel to carry that new call. The second instance is at call hand-off when the subscriber, while engaged in a call, moves from one cell in the service area to another cell, and the system selects the voice channel in the new cell that will handle the on-going call. In either case, it is important that the voice channel selected at call set-up or hand-off be of the highest quality possible.
Conventionally, the selection by the system of a voice channel in a cellular telephone system to carry a call is performed by randomly choosing a channel from the available (idle) analog or digital voice channels in a cell. Alternatively, the system maintains a record of voice channel use for each cell (normally in the form of a FIFO queue), and the voice channel selected to carry the call is the channel in the cell that has not been allocated for calling use in the longest period of time. While each of these selection methods is successful in allocating a voice channel in a cell to a mobile station for carrying the call, the selection fails to take into consideration whether the selected voice channel is the best quality (i.e., least interfered) channel available to carry the call. Thus, in many instances there may be significant interference (often comprising adjacent channel or perhaps more frequently co-channel interference) on the randomly or queue selected voice channel, and another one of the available idle voice channels in the cell would have been a more appropriate selection for use in carrying the call.
There is accordingly a need for a method and apparatus for evaluating available idle voice channel quality in a cellular telephone system prior to selecting one of those voice channels to carry a call in response to either a call set-up or hand-off situation. Such a method and apparatus should preferably be implemented without requiring alterations to existing cellular telephone system hardware, and should operate in both the conventional analog and TDMA digital environments to evaluate the quality of analog and digital voice channels, respectively.